Cladding & Roofing Fixing
Technical Bulletin #05
“How to fix roofing & cladding to Formance SIPs” is one of the most asked questions in the industry and it is easier than it looks.
Roof & wall claddings rely on the connection to the OSB, rather than the timber framing. While the 11mm thickness does not provide the same depth of fixing as timber framing, being a sheet product, fixings can be placed anywhere. Fixing strength is achieved by more frequent fixings than required for timber framing.
A wide variety of claddings can be used in conjunction with Formance SIP.
The main difference to keep in mind is that the cavity batten becomes a structural element instead of a packer. This means that products like finger-jointed battens & Cavibat cannot be used as they are non-structural.
The batten is fixed to the Formance SIP panel (in accordance with table 21 of the Formance Design Guide - pg. 47), then the cladding is fixed to the batten (in accordance with the manufacturer’s specifications).
The thickness of the cavity batten is determined by the required embedment depth of the cladding fixing. It is acceptable for the thickness of the strand board (11mm) to be added to the batten when considering embedment depths.
Follow the below steps to arrive at a solution.
- Choose the spacing of the cavity batten to suit the cladding (with reference to the cladding manufacturer specifications - for this example we will say 600mm centres).
- Find the site-specific wind zone (for this example we will use Very High).
- Determine the cladding weight (for this example we will say ‘light’)
- Use these figures as below on table 21 of pg. 47 of the Formance Design Guide.
- In this example the cavity battens need to be fixed at a maximum of 333mm centres. (it would be more practical to round it to 300mm) – This is with a 10g screw – fully threaded & embedded.
Example photos of wall cavity battens
For best practice we recommend an air gap between the purlin & the panel. This is to prevent any damming of moisture and to ensure a good air movement for keeping the cavity dry. This is especially important when the roof slope is below 15˚. A design review may be required by a ventilation specialist.
There are two solutions for air gaps for roof pitches above 15˚ that we commonly see in use,
- 45mm x 20mm H3 battens under the purlins, running 90˚ to the purlins.
- ‘Cavibat’ underneath and in line with the purlins. (Not suitable for roof pitches under 3˚).
The purlins are then fixed down through the batten into the roof panel.
There are two purlin hold-down options with Formance SIP,
- fix to the timber splines or,
- fix to the panel itself using table 22 on pg. 47 in the Formance Design Guide.
OPTION 1: Fixing purlins to the timber splines
Because most roofs are reinforced with timber splines running up the roof, this provides a double timber ‘rafter’ every 1220mm. This would be in accordance with NZS 3604 or an alternative solution such as Lumberlok provide: https://www.miteknz.co.nz/files/pdf/specification/Purlin-And-Batten-Fixing-Chart.pdf
OPTION 2: Fixing purlins to the panel
By using table 22 on page 47 of the Formance Design Guide, purlins can be fixed to the panel.
Follow the below steps.
- Ascertain the site-specific wind zone (for this example we will use Very High).
- Determine the spacing of the purlins (for this example we will say 900mm centres).
- Use these figures as shown on the right, table 22 of pg. 47 of the Formance Design Guide
- This indicates that the purlins need to be fixed at a maximum of 160mm centres, using 10g screw – fully threaded & embedded.
Other general notes regarding fixing purlins to the panel
The category ‘General’ on table 22 applies to the main roof area. The category ‘Corner’ has a higher fixing requirement due to the increased loads in the corner – re-apply the same principles as in category general.
Use the below table as a guide when calculating the size of area ‘A.
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